1. Field of the Invention
The present invention relates to a liquid crystal display apparatus having a plurality of pixel electrodes formed in a matrix pattern, and using switching devices such as thin film transistors (TFTs).
2. Description of the Related Art
A conventional matrix addressed-type liquid crystal display apparatus using switching devices such as TFTs is provided with a liquid crystal cell sandwiched between two sheets of polarizers, in which the liquid crystal cell itself is comprised at least of: a pair of transparent glass substrates disposed so as to oppose to each other; a transparent common electrode disposed on an opposing surface of one of the pair of the transparent glass substrates, and covered with an orientation film; a plurality of transparent pixel electrodes disposed in a matrix pattern on an opposing surface of the other one of the pair of the transparent glass substrates, and covered with an orientation film; TFTs each connected to each pixel electrode as a switching device; and a twisted nematic (TN) liquid crystal sealed between the two opposed orientation films. This type of liquid crystal cell is generally referred to as a twisted nematic liquid crystal display (TN-LCD).
By way of example, when applying a voltage to the pixel electrodes disposed in a matrix pattern, there is widely used such a method as the line reverse drive method whereby a voltage to be applied is reversed per each line or the column reverse drive method whereby a voltage to be applied is reversed per each column in order to be able to improve the quality of display. These drive methods, however, have such a problem-that due to a reversed application of voltage between adjacent pixel electrodes, there arises a reverse tilt domain 61, the tilt direction of which is reverse to a pre-tilt direction which is originally given to the liquid crystal, in a portion within a pixel which is arranged corresponding to each pixel electrode 62 as shown in FIG. 6 (refer to Japan Patent Publication No. 2934875, being a basic-application of U.S. Pat. No. 5,781,260 and U.S. Pat. No. 6,011,604, col. 5-6). In particular, in a normally-white mode liquid crystal display, there occurs a problem that a disclination line 63 which is located at a boundary between a normal region and the reverse tilt domain 61 is caused to have a white stripping defect thereby deteriorating contrast. Thereby, in order to reduce the region of the reverse tilt domain 61, there has been practiced to increase its tilt angle. However, this causes a problem to drop the yield at the time of manufacture. Further, in order to minimize light leakage therefrom, a shade material is disposed corresponding to a portion where reverse tilt domain 61 is formed. However, this causes a problem to drop a numerical aperture.
Thereby, as one of the methods to be able to increase a contrast ratio in the TN-LCD, it is considered promising to increase an effective voltage to be applied across its common electrode and pixel electrode which are disposed opposingly (i.e., to broaden its dynamic range). In such a case in which the effective voltage to be applied across the common electrode and the pixel electrode opposing to each other is increased, there are such advantages that the orientation of liquid crystal molecules becomes more perpendicular, and in addition that because the position of occurrence of disclination line 63 is shifted toward a peripheral (outer) direction within the pixel, the contrast ratio can be improved.
However, if this effective voltage is increased substantially, the reverse tilt domain will disappear in a part of the pixels, and because this disappearance of the reverse tilt domain in the part thereof will become a quasi-stable state, there occurs a hysteresis in V (Voltage)-T (Transmitted light strength) characteristics (refer to FIG. 7). Thereby, a significant deterioration in quality of display occurs resulting from a display defect like a dead pixel at the time of switching from black to half tone displays, or in particular, resulting from a display defect like a dead line which is caused by propagation of a display defect due to arbitrary reverse tilt domain to its adjacent reverse tilt domain. Therefore, it has not been possible to apply an effective voltage in excess of a predetermined value across the common electrode and the pixel electrode which are opposed to each other, thereby failing to realize a desired contrast ratio. These problems described above become more significant when a gap between juxtaposed pixel electrodes is reduced in order to realize a liquid crystal display device having a high resolution and a high numerical aperture. Therefore, in the matrix-addressed type liquid crystal display apparatus provided with switching devices such as TFTs, it has been difficult to accomplish a high numerical aperture and a high contrast ratio simultaneously.